The effect of a Gradual Proton Event on Mars' atmosphere is
described. The incident Solar Energetic Particles flux
crosses the planetopause/magnetopause and bow shock derived
from the interaction between Mars and the solar wind. The
low energy particles at the exobase sputter the atmosphere
generating a small loss of atmosphere but different in
comparison with the other losses due to the pick-up ions and
to the dissociative recombination of O2+ in the Martian
atmosphere. The low energy particles also generate a
significant heating of the atmosphere around the homopause
in relation with the crustal magnetic field. The high energy
particles cross the whole atmosphere, reach the surface and
are a potential danger for any human exploration. We will
describe in details these different effects with 3D models.
We will also compare these effects with the ones due to the
normal solar wind and to O+ pick-up ions. A sum-up of all
the different loss mechanisms for present epoch will be made
taking into account all the populations of incident
particles into Mars' atmosphere, of the dissociative
recombination mechanisms below the exobase and of the
photoionization above it.